As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent charge retention and service-life characteristics, which has been widely used as an energy source for various electronic products as well as various kinds of mobile devices.
Depending upon kinds of external devices in which the lithium secondary battery is used, the lithium secondary battery may be configured to have a detachable type structure in which the lithium secondary battery can be easily inserted into and removed from the external devices or to have an embedded type structure in which the lithium secondary battery is embedded in the external devices. For example, the lithium secondary battery can be inserted or removed into or from an external device, such as a tablet personal computer (PC) or a laptop computer as necessary. On the other hand, another external device, such as a smart phone or a smart pad, requires an embedded type battery pack due to the structure or capacity thereof.
Also, the above devices require a lithium secondary battery having a large capacity based on functions and characteristics thereof.
However, the capacity of the lithium secondary battery is limited as compared with the functions and characteristics of the devices.
Of course, the capacity of the lithium secondary battery may be increased in order to overcome limitations in capacity of the lithium secondary battery. If the thickness of the lithium secondary battery is increased, however, it may be difficult to apply the lithium secondary battery to the above devices, each of which has an advantage of mobility.
Alternatively, an auxiliary secondary battery exhibiting high performance may be used to overcome limitations in capacity of the lithium secondary battery. In this case, however, it is necessary to carry the auxiliary secondary battery together with the above devices. For this reason, this method is not effective to overcome limitations in capacity of the lithium secondary battery.
Meanwhile, there has been developed a means to overcome limitations in capacity of the lithium secondary battery using an environmentally friendly method.
For example, a solar charging type mobile phone has been developed. The solar charging type mobile phone is configured such that a solar light panel is provided at a liquid crystal part to continuously charge the solar charging type mobile phone without an additional charging device or a plug receptacle. However, the solar charging type mobile phone requires light having higher than predetermined luminous intensity. Also, charging time is very long. For these reasons, this method is not effective to overcome limitations in capacity of the lithium secondary battery.
As another example, a wind power type charging device has been developed. However, the wind power type charging device requires wind exceeding a predetermined velocity. Also, it is necessary to carry the wind power type charging device together with the above devices. For these reasons, this method is not effective to overcome limitations in capacity of the lithium secondary battery.
Therefore, there is a high necessity for a technology capable of increasing the capacity of the lithium secondary battery without the use of an additional auxiliary device and without the increase in volume of the existing battery pack.